Big age difference developmentally.
I have a 5 year old son.
His sister is 4 years older.
He will sometimes whine and blame her.
And then, we go and talk to him in another room. We let him tell US, why he is blaming and complaining.
To a little 5 year old's mind.... he is really valid. Because, his older sister... OFTEN is just more verbal and articulate and able to jump the gun faster than he can... in defending herself or giving reason 'why' something is unfair. Hence, he will complain/whine.
Once we KNOW... what is really going on, per his 5 year old interpretation of it.... he really is, valid. And we acknowledge, that. Then we speak to our older child. We explain how it might seem to a young 5 year old. About the situation.
Then we ALL come to an understanding of it.
This happened to my son, even just yesterday.
To him... the whole situation was really unfair. And it was. We didn't know... how he felt or WHY he was complaining. But after talking with him... calmly and by letting him vent/explain... we understood.
He was not 'wrong.'
But his manner of expressing his conundrum... was irritating.
But it was solved.
The thing is, a young child and an older child, interprets things DIFFERENTLY. Not by the same logic or emotions even.
So.
Then you also teach... your youngest child, about HOW to tell Mommy/Daddy, about what is bothering him. And show him how. Tell him the words he can use... and the tone of voice. Practice it.
My son, even at 3 years old, knew what a 'lie' was.
To him, he was not lying.
He also knows, what 'blaming' is.
But in some situations, they resort to this... because, they don't know how else to cope.